1. Field of the Invention
The present invention relates to a system for triggering the relief of a working line in a bidirectional digital transmission link which comprises a plurality of bidirectional digital working lines and at least one bidirectional digital spare line and which is interconnected between two end stations of a digital transmission system. Each end station includes a switching unit which is intended for bidirectionally switching a working line over to a spare line when a transmission fault in the working line is selected by the switching monitor of the end station.
2. Description of the Prior Art
It will be seen, right from the outset, that the invention deals, more specifically, with a procedure which triggers the relief of a faulty working line, further details of which may be found by referring to the article by A. CHOMETTE, J. GUIOMAR and M. ROBINET, entitled "Securisation et Supervision d'un reseau numerique urbain" (relieving and monitoring an urban digital network), published in the French review "L'Echo des Recherches", January 1979, pages 12 to 19, or to our other U.S. Patent Application filed as of today.
A working line in a transmission link is generally composed of bidirectional line sections which are connected by line terminals at the common ends of the line. The functions of each line terminal are ascribed to the operation and maintenance of two adjacent line sections such as the power remote-supplying and remote-locating of repeaters in the line sections. Each line terminal also monitors the transmission qualities of the line sections. With this in mind, two transmission fault detectors are incorporated in each terminal for detecting the transmission faults in relation to the digital signals travelling along the two transmission directions of the working line. A certain number of the more common transmission faults can be listed as follows: loss of clock, error rate more than a predetermined error rate, as determined by checking the line code of the received digital signal, loss of frame alignment (this is relevant to digital channels time-division multiplexed into the line and containing digital voice-mode or packet-mode signals or signalling signals), loss of the received digital signal following a break in the line section or an equipment breakdown such as a power supply failure. This equipment can be a repeater or a multiplexer-demultiplexer, which is used when certain working lines outputting of an end section are multiplexed into another working line having a higher flow rate.
Upon detection of a transmission fault for a given transmission direction, the terminal sends an alarm indication signal (AIS) out to the remote end station along the given direction. The digital data signal usually conveyed by the working line downstream of the terminal is replaced in all the multiplexed channels by the AIS which is continually emitted until the transmission fault has been removed. The alarm indication signal advises the remote switching exchange beyond the nearest end station that the working line in question is out of order.
If the line terminal which has detected a fault is a so-called end terminal, i.e. a terminal in the immediate vicinity of an end station then it delivers an alarm signal to the switching monitor of the end station via an auxiliary connection in order to trigger the switch-over from the faulty working line to a selected free spare line. This switch-over operation is performed in a known manner by exchanging digital order signals between the two end stations along the free spare line, then by making use of the switching units for passing over from the faulty working line to the previously free spare line so as to avoid virtually any break in the telephone or data transmission communications that are in progress.
Only the end terminals can trigger the switch-over procedure. In other words, the transmission faults detected solely on the line sections between the end stations and the end terminals of the link cause the switch-over. On the other hand, a transmission fault arising in a line section between two intermediary terminals, i.e. not end terminals, does not enable a working line to be relieved, thereby causing the communications in progress to be cut-off.
The transmission of an AIS via the working line of the correctly link functioning may also be taken into consideration when the section of the working line outside the link transmits an AIS in all multiplexed channels of the working line. This comes about when an equipment item outside the link, such as a channel multiplexing unit, detects a transmission fault. In this case, the working line is not switched over to the spare line since, in the link itself, this working line is operating correctly.
U.K. Pat. No. 1,558,903 discloses relieving of a digital transmission link between two end stations. The link comprises in fact only two bidirectional digital transmission lines, one playing the role of a working line and the other a spare line; the lines play these roles alternately depending on whether one or the other is faulty. Each end station includes multiplexing and demultiplexing equipment acting as a switching unit for each of the two lines, an inside alarm indication signal detector which detects an alarm indication signal conveyed by the line inside the link and an outside alarm indication signal detector for the two lines as a whole.
According to this U.K. Patent, the relief procedure for a working line transmitting the digital data signal is as follows. When there is no transmission fault in the working line, i.e. the line is functioning properly, the emitting ends in the end stations make it possible to transfer the incoming signal onto the working line and the spare line of the link simultaneously. On the other hand, the receiving ends in the end stations connect only the working line to the line section outside the link. If a fault on the working line is detected, the switch over from the working line to the spare line takes place in the receiving ends so that only the spare line retransmits the digital data signal. The same is so when the working line transmits an AIS and the spare line does not or is faulty. However, when two AIS are simultaneously transmitted by the two lines, no switching occurs.
If the two AIS arise as a result of the transmission of an outside AIS and if the working line is not faulty, there is no reason to trigger off a switch-over of the two lines. However, should the working line fail, an intermediary terminal thereof which detects a transmission fault transmits an inside AIS which is merged with the outside AIS in the end stations and these stations will not cause the switch-over. Consequently, a fault existing in the working line whilst an outside AIS is being transmitted is not indicated in the end stations, i.e. does not cause a switch-over.